Keyboard latency

If you look at “gaming” keyboards, a lot of them sell for $100 or more on the promise that they’re fast. Ad copy that you’ll see includes:

• a custom designed keycap that has been made shorter to reduce the time it takes for your actions to register
• 8x FASTER - Polling Rate of 1000Hz: Response time 0.1 milliseconds
• Wield the ultimate performance advantage over your opponents with light operation 45g key switches and an actuation 40% faster than standard Cherry MX Red switches
• World’s Fastest Ultra Polling 1000Hz
• World’s Fastest Gaming Keyboard, 1000Hz Polling Rate, 0.001 Second Response Time

Despite all of these claims, I can only find one person who’s publicly benchmarked keyboard latency and they only tested two keyboards. In general, my belief is that if someone makes performance claims without benchmarks, the claims probably aren’t true, just like how code that isn’t tested (or otherwise verified) should be assumed broken.

The situation with gaming keyboards reminds me a lot of talking to car salesmen:

Salesman: this car is super safe! It has 12 airbags! Me: that’s nice, but how does it fare in crash tests? Salesman: 12 airbags!

Sure, gaming keyboards have 1000Hz polling, but so what?

Two obvious questions are:

1. Does keyboard latency matter?
2. Are gaming keyboards actually quicker than other keyboards?

Does keyboard latency matter?

A year ago, if you’d asked me if I was going to build a custom setup to measure keyboard latency, I would have said that’s silly, and yet here I am, measuring keyboard latency with a logic analyzer.

It all started because I had this feeling that some old computers feel much more responsive than modern machines. For example, an iMac G4 running macOS 9 or an Apple 2 both feel quicker than my 4.2 GHz Kaby Lake system. I never trust feelings like this because there’s decades of research showing that users often have feelings that are the literal opposite of reality, so got a high-speed camera and started measuring actual keypress-to-screen-update latency as well as mouse-move-to-screen-update latency. It turns out the machines that feel quick are actually quick, much quicker than my modern computer – computers from the 70s and 80s commonly have keypress-to-screen-update latencies in the 30ms to 50ms range out of the box, whereas modern computers are often in the 100ms to 200ms range when you press a key in a terminal. It’s possible to get down to the 50ms range in well optimized games with a fancy gaming setup, and there’s one particularly impressive consumer device that can easily get below that. But in general, the default experience is much slower. Modern computers have much better throughput, but their latency isn’t so great.

Anyway, at the time I did these measurements, my 4.2 GHz kaby lake had the fastest single-threaded performance of any machine you could buy but had worse latency than a quick machine from the 70s (roughly 6x worse than an Apple 2), which seems a bit curious. To figure out where the latency comes from, I started measuring keyboard latency because that’s the first part of the pipeline. My plan was to look at the end-to-end pipeline and start at the beginning, ruling out keyboard latency as a real source of latency. But it turns out keyboard latency is significant! I was surprised to find that the median keyboard I tested has more latency than the entire end-to-end pipeline of the Apple 2. If this doesn’t immedately strike you as absurd, consider that an Apple 2 has 3500 transistors running at 1MHz and an Atmel employee estimates that the core used in a number of high-end keyboards today has 80k transistors running at 16MHz. That’s 20x the transistors running at 16x the clock speed – keyboards are often more powerful than entire computers from the 70s and 80s! And yet, the median keyboard today adds as much latency as the entire end-to-end pipeline as a fast machine from the 70s.

Let’s look at the measured keypress-to-USB latency on some keyboards:

keyboard	latency (ms)	connection	gaming
apple magic (usb)	15	USB FS
MS natural 4000	20	USB
das 3	25	USB
logitech k120	30	USB
filco majestouch	30	USB
dell OEM	30	USB
powerspec OEM	30	USB
razer ornata chroma	35	USB FS	Yes
olkb planck rev 4	40	USB FS
ergodox	40	USB FS
MS comfort 5000	40	wireless
easterntimes i500	50	USB FS	Yes
kinesis advantage	50	USB FS
genius luxemate i200	55	USB
topre type heaven	55	USB FS
logitech k360	60	“unifying”

The latency measurements are the time from when the key starts moving to the time when the USB packet associated with the key makes it out onto the USB bus. Numbers are rounded to the nearest 5 ms in order to avoid giving a false sense of precision. The easterntimes i500 is also sold as the tomoko MMC023.

The connection column indicates the connection used. USB FS stands for the usb full speed protocol, which allows up to 1000Hz polling, a feature commonly advertised by high-end keyboards. USB is the usb low speed protocol, which is the protocol most keyboards use. The ‘gaming’ column indicates whether or not the keyboard is branded as a gaming keyboard. wireless indicates some kind of keyboard-specific dongle and unifying is logitech’s wireless device standard.

We can see that, even with the limited set of keyboards tested, there can be as much as a 45ms difference in latency between keyboards. Moreover, a modern computer with one of the slower keyboards attached can’t possibly be as responsive as a quick machine from the 70s or 80s because the keyboard alone is slower than the entire response pipeline of some older computers.

That establishes the fact that modern keyboards contribute to the latency bloat we’ve seen over the past forty years. The other half of the question is, does the latency added by a modern keyboard actually make a difference to users? From looking at the table, we can see that among the keyboard tested, we can get up to a 40ms difference in average latency. Is 40ms of latency noticeable? Let’s take a look at some latency measurements for keyboards and then look at the empirical research on how much latency users notice.

There’s a fair amount of empirical evidence on this and we can see that, for very simple tasks, people can percieve latencies down to 2ms or less. Moreover, increasing latency is not only noticable to users, it causes users to execute simple tasks less accurately. If you want a visual demonstration of what latency looks like and you don’t have a super-fast old computer lying around, check out this MSR demo on touchscreen latency.

Are gaming keyboards faster than other keyboards?

I’d really like to test more keyboards before making a strong claim, but from the preliminary tests here, it appears that gaming keyboards aren’t generally faster than non-gaming keyboards.

Gaming keyboards often claim to have features that reduce latency, like connecting over USB FS and using 1000Hz polling. The USB low speed spec states that the minimum time between packets is 10ms, or 100 Hz. However, it’s common to see USB devices round this down to the nearest power of two and run at 8ms, or 125Hz. With 8ms polling, the average latency added from having to wait until the next polling interval is 4ms. With 1ms polling, the average latency from USB polling is 0.5ms, giving us a 3.5ms delta. While that might be a significant contribution to latency for a quick keyboard like the Apple magic keyboard, it’s clear that other factors dominate keyboard latency for most keyboards and that the gaming keyboards tested here are so slow that shaving off 3.5ms won’t save them.

Conclusion

Most keyboards add enough latency to make the user experience noticeably worse, and keyboards that advertise speed aren’t necessarily faster. The two gaming keyboards we measured weren’t faster than non-gaming keyboards, and the fastest keyboard measured was a minimalist keyboard from Apple that’s marketed more on design than speed.

Previously, we’ve seen that terminals can add significant latency, up 100ms in mildly pessimistic conditions. In a future post, we’ll look at the entire end-to-end pipeline to see other places latency has crept in and we’ll also look at how some modern devices keep latency down.

Appendix: where is the latency coming from?

A major source of latency is key travel time. It’s not a coincidence that the quickest keyboard measured also has the shortest key travel distance by a large margin. video setup I’m using to measure end-to-end latency is a 240 fps camera, which means that frames are 4ms apart. When videoing “normal” keypresses and typing, it takes 4-8 frames for a key to become fully depressed. Most switches will start firing before the key is fully depressed, but the key travel time is still significant and can easily add 10ms of delay (or more, depending on the switch mechanism). Contrast this to the Apple keyboard measured, where the key travel is so short that it can’t be captured with a 240 fps camera, indicating that the key travel time is < 4ms.

Note that, unlike the other measurement I was able to find online, this measurement was from the start of the keypress instead of the switch activation. This is because, as a human, you don’t activate the switch, you press the key. A measurement that starts from switch activiation time misses this large component to latency. If, for example, you’re playing a game and you switch from moving forward to moving backwards when you see something happen, you have pay the cost of the key movement, which is different for different keyboards. A common response to this is that “real” gamers will preload keys so that they don’t have to pay the key travel cost, but if you go around with a high speed camera and look at how people actually use their keyboards, the fraction of keypresses that are significantly preloaded is basically zero even when you look at gamers. It’s possible you’d see something different if you look at high-level competitive gamers, but even then, just for example, people who use a standard wasd or esdf layout will typically not preload a key when going from back to forward. Also, the idea that it’s fine that keys have a bunch of useless travel because you can pre-depress the key before really pressing the key is just absurd. That’s like saying latency on modern computers is fine because some people build gaming boxes that, when run with unusually well optimzed software, get 50ms response time. Normal, non-hardcore-gaming users simply aren’t going to do this. Since that’s the vast majority of the market, even if all “serious” gamers did this, that would stll be a round error.

The other large sources of latency are scaning the keyboard matrix and debouncing. Neither of these delays are inherent – keyboards use a matrix that has to be scanned instead of having a wire per-key because it saves a few bucks, and most keyboards scan the matrix at such a slow rate that it induces human noticable delays because that saves a few bucks, but a manufacturer willing to spend a bit more on manufacturing a keyboard could make the delay from that far below the threshold of human perception. See below for debouncing delay.

Appendix: counter-arguments to common arguments that latency doesn’t matter

Before writing this up, I read what I could find about latency and it was hard to find non-specialist articles or comment sections that didn’t have at least one of the arguments listed below:

Computers and devices are fast

The most common response to questions about latency is that input latency is basically zero, or so close to zero that it’s a rounding error. For example, two of the top comments on this slashdot post asking about keyboard latency are that keyboards are so fast that keyboard speed doesn’t matter. One person even says

There is not a single modern keyboard that has 50ms latency. You (humans) have that sort of latency.

As far as response times, all you need to do is increase the poll time on the USB stack

As we’ve seen, some devices do have latencies in the 50ms range. This quote as well as other comments in the thread illustrate another common fallacy – that input devices are limited by the speed of the USB polling. While that’s technically possible, most devices are nowhere near being fast enough to be limited by USB polling latency.

Unfortunately, most online explanations of input latency assume that the USB bus is the limiting factor.

Humans can’t notice 100ms or 200ms latency

Here’s a “cognitive neuroscientist who studies visual perception and cognition” who refers to the fact that human reaction time is roughly 200ms, and then throws in a bunch more scientific mumbo jumbo to say that no one could really notice latencies below 100ms. This is a little unusual in that the commenter claims some kind of special authority and uses a lot of terminology, but it’s common to hear people claim that you can’t notice 50ms or 100ms of latency because human reaction time is 200ms. This doesn’t actually make sense because there are independent quantities. This line of argument is like saying that you wouldn’t notice a flight being delayed by an hour because the duration of the flight is six hours.

Another problem with this line of reasoning is that the full pipeline from keypress to screen update is quite long and if you say that it’s always fine to add 10ms here and 10ms there, you end up with a much larger amount of bloat through the entire pipeline, which is how we got where we are today, where can buy a system with the CPU that gives you the fastest single-threaded performance money can buy and get 6x the latency of a machine from the 70s.

It doesn’t matter because the game loop runs at 60 Hz

This is fundamentally the same fallacy as above. If you have a delay that’s half the duration a clock period, there’s a 50% chance the delay will push the event into the next processing step. That’s better than a 100% chance, but it’s not clear to me why people think that you’d need a delay as long as the the clock period for the delay to matter. And for reference, the 45ms delta between the slowest and fastest keyboard measured here corresponds to 2.7 frames at 60fps.

Keyboards can’t possibly response faster more quickly than 5ms/10ms/20ms due to debouncing

Even without going through contortions to optimize the switch mechanism, if you’re willing to put hysteresis into the system, there’s no reason that the keyboard can’t assume a keypress (or release) is happening the moment it sees an edge. This is commonly done for other types of systems and AFAICT there’s no reason keyboards couldn’t do the same thing (and perhaps some do). The debounce time might limit the repeat rate of the key, but there’s no inherent reason that it has to affect the latency. And if we’re looking at the repeat rate, imagine we have a 5ms limit on the rate of change of the key state due to introducing hysteresis. That gives us one full keypress cycle (press and release) every 10ms, or 100 keypresses per second per key, which is well beyond the capacity of any human. You might argue that this introduces a kind of imprecision, which might matter in some applications (music, rythym games), but that’s limited by the switch mechanism. Using a debouncing mechanism with hysteresis doesn’t make us any worse off than we were before.

Apologies for not explaining terminology here, but I think that anyone making this objection should understand the explanation :-).

Appendix: experimental setup

The USB measurement setup was a USB cable that was cut open to expose the wires, connected to a logic analyzer. The exact model of logic analzyer doesn’t really matter, but if you’re curious about the details, this set of experiments used a salae pro.

The start-of-input was measured by pressing two keys at once – one key on the keyboard and a button that was also connected to the logic analyzer. This introduces some jitter as the two buttons won’t be pressed at exactly the same time. To calibrate the setup, we used two identical buttons connected to the logic analyzer. The median jitter was < 1ms and the 90%-ile jitter was roughly 5ms. This is enough that tail latency measurements for quick keyboards aren’t really possible with this setup, but average latency measurements like the ones done here seem like they should be ok. The input jitter could probably be reduced to a negligible level by building a device to both trigger the logic analyzer and press a key on the keyboard under test at the same time. Average latency measurements would also get better with such a setup (because it would be easier to run a large number of measurements).

If you want to know the exact setup, a E-switch LL1105AF065Q switch was used. Power and ground were supplied by an arduino board. There’s no particular reason to use this setup. In fact, it’s a bit absurd to use an entire arduino to provide power, but this was done with spare parts that were lying around and this stuff just happened to be stuff that RC had in their lab, with the exception of the switches. There weren’t two identical copies of any switch, so we bought a few switches so we could do calibration measurements with two identical switches. The exact type of switch isn’t important here; any low-resistance switch would do.

Tests were done by pressing the z key and then looking for byte 29 on the USB bus and then marking the end of the first packet containing the appropriate information. But, as above, any key would do.

I would love to test more keyboards! If you’re in the NYC area and have a keyboard that I can borrow for a test, please contact me! I’m also looking for more phones and machines to measure and would love to run a quick benchmark with a high-speed camera if you have a machine or device that’s not on this list! If you’re not in the area and want to donate a keyboard for me to test, feel free to mail the keyboard to

Dan Luu
Recurse Center
455 Broadway, 2nd Floor
New York, NY 10013

The setup I currently have can only get measurements from devices that present themselves as standard USB keyboards devices, but I might try to decode keyboards that use other protocols over USB and I might also try to build a setup that can handle ps/2 sometime.

This project was done with help from Wesley Aptekar-Cassels, Leah Hanson, and Kate Murphy. BTW, Wesley is looking for work. In addition to knowing “normal” programming stuff, he’s also familiar with robotics, controls, electronics, and general “low-level” programming.

Thanks to RC, Ahmad Jarara, Raph Levien, Peter Bhat Harkins, Brennan Chesley, Dan Bentley, Kate Murphy, Christian Ternus, and Dan Puttick, for letting us use their keyboards for testing.

Thanks for Leah Hanson, Mark Feeney, and Zach Allaun for comments/corrections/discussion on this post.

Read the full article here by Dan Luu